Rare earth metal-based permanent magnets, for instance, R—Fe—B based permanent magnets represented by a Nd—Fe—B based permanent magnet, or R—Fe—N based permanent magnets represented by a Sm—Fe—N based permanent magnet, etc., utilize inexpensive materials abundant in resources and possess superior magnetic characteristics; particularly among them, the R—Fe—B based permanent magnets are employed today in various fields.
However, since rare earth metal-based permanent magnets contain a highly reactive rare earth metal: R, they are apt to be oxidized and corroded in ambient, and in case they are used without applying any surface treatment, corrosion tends to proceed from the surface in the presence of small acidic or alkaline substance or water to generate rust, and this brings about the degradation and the fluctuation of magnetic characteristics. Moreover, in case such a rusty magnet is embedded in a magnetic circuit and a like device, there is fear of scattering rust as to contaminate peripheral components.
In the light of above-mentioned circumstances, there has been employed a method for forming an Al film by a vapor phase plating method such as a vapor deposition method, on the surface of a rare earth metal-based permanent magnet for the purpose of providing an excellent corrosion resistance to the magnet. Because an Al film not only has an excellent corrosion resistance but also has a superior adhesion reliability with the adhesive which is necessary in embedding the components (delamination hardly occurs between the film and the adhesive up to the intrinsic fracture strength of the adhesive), the Al film is widely applied to rare earth metal-based permanent magnets where high adhesion strength is required, and the rare earth metal-based permanent magnets having an Al film on the surface thereof are utilized by embedding them in various types of motors.
The rare earth metal-based permanent magnets being embedded in automotive motors, which, among various types of motors, may be regarded as magnets used under the severest utilization environments, because they are used under environments with violent temperature change, or they are exposed to chlorine ions contained in antifreezing agents that are spread on roads in cold regions, or they are exposed to salt water in the coastal area. Accordingly, the rare earth metal-based permanent magnets being embedded in automotive motors are required to exhibit an excellent corrosion resistance even in the severest corrosion resistance test, i.e., a salt water spray test. Regretfully, however, Al film is not sufficiently resistant against salt water. As a method for improving a salt water resistance of a rare earth metal-based permanent magnet having an Al film on the surface thereof, there can be mentioned methods such as forming laminates of chemical conversion films (Patent literature 1), or metal oxide films (Patent literature 2) on the surface of the Al film; however, there still are problems such that it makes the production process complicated, or that the salt water resistance is still insufficient.
Patent literature 3 discloses forming a film containing Mg and oxygen on the plated surface of a Al hot-dipping plated steel sheet under open atmosphere, by which a sufficient corrosion resistance is obtained under a salt corrosion environment, and as a method for forming such a film, there is mentioned a method comprising conducting Al hot-dipping by immersing the steel sheet into a bath containing Al and Mg, followed by allowing the dipped steel sheet to stand in ambient atmosphere to effect surface oxidation. However, this method cannot be applied to rare earth metal-based permanent magnets due to the following reasons.                If Al hot-dipping, which is carried out at a temperature of 500° C. or higher, should be applied to magnets, the surface of the magnets reacts with Al and undergoes changing in quality under such a high temperature as to deteriorate the magnetic characteristics.        The changing in quality of the surface of a magnet exerts considerable harmful effects particularly on the magnetic characteristics of compact magnets.        Contact imprints remain on the magnets because hot-dipping basically comprises immersing and drawing out processes.        
Accordingly, as a method for imparting a salt water resistance to a rare earth metal-based permanent magnet, one of the present inventors has proposed in patent literature 4 a method comprising vapor depositing an Al film containing 3 mass % to 10 mass % of Mg on the surface of a magnet.
Patent Literature 1: JP-A-2000-150216
Patent Literature 2: JP-A-2000-232011
Patent Literature 3: JP-A-2000-282262
Patent Literature 4: JP-A-2005-191276